HD 217786

HD 217786 is a binary star^[4] system in the equatorial constellation of Pisces. With an apparent visual magnitude of 7.78,^[1] it requires binoculars or a small telescope to view. The system is located at a distance of 181 light-years from the Sun based on parallax, and is drifting further away with a radial velocity of +10 km/s.^[1] Kinematically, the star system belongs to the thin disk population of the Milky Way.^[7]

HD 217786

Observation data Epoch J2000      Equinox J2000
Constellation	Pisces[1]
Right ascension	23h 03m 08.20704s[2]
Declination	−00° 25′ 46.6777″[2]
Apparent magnitude (V)	7.78[1]
Characteristics
Spectral type	F8V[3]
B−V color index	0.578±0.004[1]
Astrometry
Radial velocity (Rv)	+10.00±0.02[1] km/s
Proper motion (μ)	RA: −89.933[2] mas/yr Dec.: −168.781[2] mas/yr
Parallax (π)	17.9946±0.0793 mas[2]
Distance	181.3 ± 0.8 ly (55.6 ± 0.2 pc)
Absolute magnitude (MV)	4.09[1] + 11.02±0.13[4]
Details[5]
A
Mass	1.02 M☉
Radius	1.32±0.06 R☉
Luminosity	1.93±0.04[6] L☉
Surface gravity (log g)	4.13±0.02 cgs
Temperature	5,882±8[7] K
Metallicity	−0.19±0.01
Rotational velocity (v sin i)	1.18±0.05 km/s
Age	9.40±0.22 Gyr
HD 217786 B
Mass	0.1622+0.0071 −0.0068[4] M☉
Other designations
BD−01°4382, Gaia DR2 2650902026099857920, HD 217786, HIP 113834, TYC 5242-591-1, GSC 05242-00591, 2MASS J23030822-0025465[8]
Database references
SIMBAD	data

The primary is an F-type main-sequence star with a stellar classification of F8V. It is much older than Sun with an estimated age of 9.4 billion years and is spinning slowly with a projected rotational velocity of 1.2 km/s. The star has a lower proportion of heavy elements than the Sun, having 65% of solar abundance.^[7] It has about the same mass as the Sun but a 32% larger radius.^[5] The star is radiating nearly double^[6] the luminosity of the Sun from its photosphere at an effective temperature of 5,882 K.^[7]

A low-mass stellar companion at a projected separation of 155 AU was discovered in 2016.^[4] The proper motion of this co-moving object suggests it is gravitationally-bound to the primary, and their orbit is being viewed edge-on. If the orbit is assumed to be circular, then the orbital period for the pair is ~6.2 Myr.^[4] No other companion stars have been detected at separations from 2.74 to 76.80 AUs.^[6]

The star system exhibits strong stellar flare activity in the ultraviolet.^[9]

Planetary system

In 2010 one superjovian planet or brown dwarf on an eccentric orbit was discovered utilising the radial velocity method.^[10] Designated component Ab, the high eccentricity of this object may have been caused by interaction with the secondary star.^[4] In 2022, the inclination and true mass of HD 217786 Ab were measured via astrometry, and a second planet was discovered orbiting closer to the star.^[11]

The HD 217786 A planetary system^[11]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
c	≥0.023±0.002 MJ	0.038±0.002	2.5+0.00010 −0.00005	—	—	—
b	13.852+1.267 −1.311 MJ	2.446+0.109 −0.119	1,301.4+1.2 −0.5	0.311+0.002 −0.003	69.767+0.601 −0.386°	—